Noisy transformer

[akis]

Last night I powered up the 300 VA toroidal and it is making a quiet hum/buzz. On smaller toroidals this usually means there is a short somewhere, but in my case the buzz persists with the transformer bolted down, or with bolt removed, at a different location, and with bridge+filters connected or not.

Any idea what could be causing this noise?

[runebivrin]

Small DC component in the mains. You may need a DC blocker circuit.

[akis]

Never heard about DC component in mains. What could be the cause of that?

[peranders]

DC on the mains have we discussed quite much here.

Use Google search here and use "DC trap" and "DC mains" as keywords.

You can get DC on the mains if you for example connect a hair drier at half speed. By that you will get an unsymmetrical sinus wave.

[Speedskater]

Before you start chasing DC ghosts, try your transformer at someone else's house. It's not easy to get real DC current on an AC line in the modern world.

[Glowbug]

The Decware CSP I had had the worst lamination buzz I've heard. Nothing else I've used does it here, oddly enough.

[wg_ski]

Originally Posted by Speedskater

Before you start chasing DC ghosts, try your transformer at someone else's house. It's not easy to get real DC current on an AC line in the modern world.

The total DC current at the pole transformer would need to be zero. But that doesn't mean it cant exist downstream. If one leg pulls a DC current, you get an equal and opposite one forced in the other.

But it's just as likely a high line voltage. It can run 130V (in the US), even if everything is in spec. Get a loose or resistive neutral connection in your breaker box, the two legs can get forced to be unsymmetrical and then it can go really high on one side. You may want to try it at someone else's house - espicially if you've got other symptoms like lights going bright and dim.

[akis]

There are two smaller toroidals (50VA and 80VA) which are not making this buzzing noise (unless you pull like 90% of their rated current).

There are two fluorescent tubes transformers which are making similar buzzing noise, I suppose for different reasons. The soldering iron station also buzzes when it heats. I presume all those symptoms are unrelated.

The buzzing is mechanical, something is vibrating. Could it be the core is fragmented? Could it be there is a few loose windings ? In either case could this mean the transformer is unsafe and a potential risk?

[runebivrin]

It could be a mechanical issue in the transformer. That happens, and isn't something to be overly concerned about (other than the sound being obnouxious). However, a very small DC component on the mains will easily saturate the core, which results in larger current draw and leaking magnetic field. The larger the transformer the bigger the issue.

Two large low-voltage electrolytics is enough to test that hypothesis. Add two diodes for continous use.

[TechGuy]

If is a DC or a saturation issue you should see a measuable currect at the input with no load attached. It may just be that they didn't properly laminate the transformer plates during manufacturing. If that is the case you could request a replacement from the manufacturer or distributor. Another issue could be a short in the primary winding. Try measuring the input inductance to and compare it with the smaller non-buzzing transformers. I would expect the larger transformer to have a slightly lower input inductance. But if its really low then there is a problem.

You can also try vacuum impregnating the transformer (by submerging the entire transformer in varnish inside of a vacuum desiccator) to correct a bad lamination job. By creating a vacuum all of the air is removed from inside the transformer and is replaced with varnish. When the varnish sets it should imobilize everything, preventing vibration noise.